1. Field of the Invention
This invention relates to electronic amplifiers generally, and more specifically to a wideband, distributed integrated-circuit microwave amplifier suitable for use in fiber-optic and similar broadband communications applications.
2. Description of the Related Art
In broadband fiberoptics and similar communication applications, the choice of signaling waveform used can fundamentally impact the quality of the communication link. For example, in many intermediate and long reach fiber optic links, the so-called xe2x80x9creturn to zeroxe2x80x9d (RZ) binary signaling technique is used in which the signal level turns on and off within one clock cycle for a logical one and remains zero for a logical zero. The fiber optic medium and other transmission media exhibit certain waveform distortion mechanisms such as chromatic dispersion, polarization mode dispersion (PMD), and non-linear distortion, all of which severely degrade the signal to noise ratio of the communication channel (with the impairment typically increasing with distance).
The interaction of the lightwave with the fiber optic medium results in a preference for certain waveforms which are able to mitigate or even minimize pulse broadening effects. An extreme case is the xe2x80x9csolitonxe2x80x9d in which nonlinear interactions completely cancel pulse broadening effects of some kinds of dispersion. Even when the soliton effect is not present, the shape (or equivalently, spectral content) of the constituent pulse waveforms significantly affects both transmitter and the receiver. However, a unique and optimal pulse shaping strategy cannot be generalized for all communication environments, because links of varying lengths and characteristics demand different pulse shaping strategies. Even within the same network, various optical fibers may be encountered due to juxtaposition of newer, high performance fibers with xe2x80x9clegacyxe2x80x9d fibers which have not been upgraded.
A simple strategy is desired for obtaining properly shaped pulses in a programmable way to implement transmitter and receiver modules that can operate over a variety of transmission environments (e.g., links of varying lengths and therefore varying dispersion effects). To be practical, any such strategy should be capable of operation at frequencies in excess of 10 GHz (and indeed, desirably at frequencies above 50 GHz) to cope with the data rates and spectral content of current and planned optical networks.
It is known that a distributed amplifier is advantageous for handling extreme bandwidths required for data rates of 10 Gigabits per second and beyond. The distributed amplifier is also known as a traveling wave amplifier because transmission lines and propagating input and output waves are integral to the operation of the device. In a traveling wave amplifier, as shown in FIG. 1, the input signal 20 traverses a transmission line 22 periodically loaded by the inputs of amplifying unit cells 24a-24h (collectively, 24). The output signal 26 likewise traverses a transmission line 28 periodically loaded by the output ports of the amplifying unit cells 24a-24h. Each unit cell samples a portion of the input xe2x80x9ctraveling wave,xe2x80x9d amplifies it and injects the amplified output signal onto the output transmission line. The unused backward wave on the output transmission line 28 is dissipated in a matched load 30. The forward wave, which is the sum of individually amplified signals, is delivered to the load 32.
Conventional distributed amplifiers have been developed which operate in the regions above 10 GHz with exceptionally wide bandwidth. However, they are generally difficult to fabricate, tend toward instability, and have fixed bandwidth (which often varies among xe2x80x9cidenticalxe2x80x9d chips due to process variation).
In view of the above problems, an apparatus of the present invention is a distributed integrated amplifier with variable bandwidth, suitable for use at Gigahertz frequencies. The amplifier circuit includes an input transmission line; an output transmission line; a plurality of amplifier cells having a respective plurality of cell inputs distributed along the input transmission line and cell outputs distributed along said output transmission line. Each amplifier cell comprises a cascode amplifier having an input transistor, an output transistor, and a variable impedance device in a circuit branch coupled to a gate of the output transistor. The impedance of the variable impedance device is responsive to a variable bias control signal.
In its method aspect the invention includes a method of improving quality of a dispersive communication channel by varying the bandwidth of at least one of a variable bandwidth transmitter amplifier and a variable bandwidth receiver amplifier.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings, in which: